Definitions of alto:

• noun:   the pitch range of the lowest female voice
• noun:   a singer whose voice lies in the alto clef
• noun:   the highest adult male singing voice
• noun:   the lowest female singing voice
• adjective:   (of a musical instrument) second highest member of a group

  Example: "Alto clarinet or recorder"

• adjective:   of or being the lowest female voice
• adjective:   of or being the highest male voice; having a range above that of tenor
• name:  A surname (very rare: popularity rank in the U.S.: #23257)